The EuroNight Jan Kiepura takes you directly from Amsterdam in the Netherlands to Warsaw in Poland. This is by no means a short journey, since you will cross all the way through Germany and the larger part of Poland. With the Jan Kiepura you can cover much ground in one night and wake up in completely different surroundings.
Please keep in mind that the Eurail Global Pass isn't valid for train travel in Poland. You will need to pay an additional fee for that part of the journey.